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306概率简单数值题short

Robust Intermediate Positions in a Permutation

题目

Let σ\sigma be a uniformly random permutation of {1,2,,n}\{1, 2, \dots, n\}. Call position i{2,,n1}i \in \{2, \dots, n-1\} an intermediate position if σ(i)\sigma(i) is strictly between σ(i1)\sigma(i-1) and σ(i+1)\sigma(i+1), i.e., min(σ(i1),σ(i+1))<σ(i)<max(σ(i1),σ(i+1))\min(\sigma(i-1), \sigma(i+1)) < \sigma(i) < \max(\sigma(i-1), \sigma(i+1)). What is the expected number of intermediate positions?

Additional robustness twist: before observation, an independent random relabeling of outcome labels is applied. Compute the same target and justify invariance.

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